1. Field of the invention
The present invention relates to a heat transfer device for sheet assembly or a laminator, wherein the sheet assembly comprising a transfer sheet on which an image has been formed and a receptor sheet for printing superimposed on an auxiliary plate passes between a pair of heat pressure rollers so as to heat transfer the image on the transfer sheet onto the receptor sheet for printing.
2. Description of the Related Art
When an image is digitally processed and the image is transferred onto a transfer sheet with a printer based on the digital data, an image with a rich tone can be reproduced, realizing a sharp image quality. Since the tone of the image obtained a printed matter obtained by the offset printing of the image on the transfer sheet is slightly different from the tone of the image on the transfer sheet, a printing sample applied with color correction beforehand is necessary in order to grasp the finished image on the printed matter accurately.
As a device for producing the printing sample, recent attention is paid to a heat transfer device for sheet assembly (laminator). The heat transfer device for sheet assembly is constructed to transfer an image on a transfer sheet 100 onto a receptor sheet for printing by inserting a sheet assembly 108 comprising a transfer sheet 100 on which an image has been formed and a receptor sheet for printing 102 superimposed on an auxiliary plate 104 of an aluminum plate of about 0.3 mm thickness, further applied with a cover sheet 106 thereon, from an inlet opening formed on a casing along an inlet stand, and passing between a pair of heat pressure rollers. By the use of an ink ribbon applied with a thin film containing a pigment matched with the hue of a printing ink for printing an image on a transfer sheet based on digital data, a printing sample excellently exact to the actual printed matter at the time of the heat transfer of the image on the transfer sheet on the receptor sheet for printing can be provided stably.
In the heat transfer device for sheet assembly, the pair of heat pressure rolls comprise soft rollers comprising core rollers of metal pipes, of which surface covered with a soft synthetic resin such as silicone rubber, and built-inheat lamps. In order to obtain a printing sample exact to the actual printed matter, it is necessary to have the temperature of the heat pressure rollers homogeneously as much as possible at the time of the heat transfer by pressing the transfer sheet onto the receptor sheet for printing with the heat pressure rollers. If the temperature of the heat pressure rollers rises during the heat transfer process, not only inconveniences occur such as uneven transfer and generation of wrinkles in the receptor sheet for printing due to difficulty of peeling off the transfer sheet and the receptor sheet for printing, but also deterioration of the heat pressure rollers due to the temperature higher than the bearable temperature of the soft synthetic resin.
Therefore, the temperature of the heat pressure rollers is measured by a sensor for thyristor control of the power inputted to the heater lamps arranged in the heat pressure rollers. In order to prevent casting heat effect on other electric control parts in the casing, the temperature rise of the heat pressure rollers is prevented by covering the heat pressure rollers with roller covers as well as providing an exhaust fan at the bottom of the casing for taking cooling air from the inlet opening and cooling the heat pressure rollers.
Since the transfer sheet and the receptor sheet for printing adsorb moisture, the moisture may be evaporated by the heat of the heat pressure rollers so that the sheet assembly curls at the time of inputting from the inlet opening formed on the casing along the inlet stand. As to the auxiliary plate, even though it comprises an aluminum plate, in the case the thickness is about 0.3 mm, "warpage" would generate over a long term use. Accordingly, the sheet assembly may contact to the upper edge or surface of the inlet opening and choke the inlet opening due to deformation. In this case, intake of the cooling air from the inlet opening is bothered, resulting in extraordinary temperature rise of the heat pressure rollers.